Module 5: RECRYSTALLIZATION AND SUBLIMATION

• also known as fractional crystallization

• involves the dissolution of the solid in an appropriate solvent at an elevated temperature and the subsequent “re-formation of the crystals upon cooling”, so that any impurities remain in solution

• desired compound should be ____________________________

RECRYSTALLIZATION PROCESS

_____________ should either be insoluble in the solvent at all temperatures or must remain at least moderately soluble in the cold solvent

• “most critical step” in the process: selected to form a product of high purity and in good recovery or yield

• the best solvent will dissolve the solute in hot solution but not in the cold solution); vice versa, it is called a poor solvent

the boiling point of the solvent should be low enough so that it can readily be removed from the crystals (True or False)

• the solvent should not react chemically with the substance being purified (True or False)

• most ______________ dissolve in hot solvents than in cold ones

high degree of microporosity; has large surface area

▪ using excessive amounts may decrease recovery; use “dropwise method”

too less solvent will not yield crystals

“removes high molecular weight impurities”, which are often colored and relatively less soluble

• a ______________ should be used to minimize crystallization in the funnel

• sometimes the sample is ______________ with small quantity of insoluble impurity that will never dissolve

• using ______________ will minimize crystallization on the filter

• it is important to filter the solution while hot to prevent _____________ during filtration

reduce the temperature to induce crystal formation; generally the solution should not be disturbed as it cools, since this also leads to production of small crystals

crystal of the original solid is added to the solution to induce crystallization, which may then be quite rapid

▪ _____________ the solution will induce bumping of crystals which causes to reformation of more crystals

a glass rod is to rub the inside surface of the crystallization vessel at or just above the air/solution interface; microscopic scratches provide sharp edges where crystals may grow

• during cooling and crystallization, the solution should be protected from _________________ by covering the opening with a piece of filter paper, an inverted beaker, or by loosely plugging it with a clean cork

solution should not be ___________ as it cools